Motion Perception

The primate brain has visual receptor systems dedicated to motion detection (Lisberger and Movshon, 1999) which humans use to perceive movement (Nakayama and Tyler, 1981). Whereas visual velocity (i.e., speed and direction of a visual stimulus on the retina) is sensed directly in primates and humans, visual acceleration is not, but is instead inferred through processing of velocity signals (Lisberger and Movshon, 1999). Most visual-perception scientists agree that for motion perception, visual acceleration is not as important as visual velocity (Regan et al., 1986). Thus, I assume scene velocity is a better measure of scene-motion thresholds than scene acceleration.

2.2.7.2 Object-Relative versus Subject-Relative Judgments

Judgment of motion is object-relative when an object is judged to move relative to another object. Object-relative judgments depend solely upon stimuli on the retina and do not take into account extra-retinal information such as eye or head motion. Which object is moving is ambiguous.

Judgment of motion is subject-relative when an object is judged to move relative to the observer. This egocentric frame of reference is provided by extra-retinal information such as vestibular input. This egocentric frame may be the body (body-centric), the head (head-centric), or the eyes (ocular-centric).

Subject-relative cues always exist. Even when the head is held still, the brain still receives input from the vestibular system that the head is not moving. Thus object-relative cues cannot occur in isolation—when object-relative cues are present then subject-relative cues are also present. A subject-relative cue without any object- relative cues occurs only when a single visual stimulus is visible or all visuals in the scene have the same motion.

People use both object-relative and subject-relative cues to judge motion of the external world. Humans are much more sensitive to object-relative cues than subject- relative cues (Mack, 1986). Incorrectly moving visuals are more easily noticed in optical- see-through HMDs (i.e., augmented reality) than non-see-through HMDs (Azuma, 1997), since users can directly compare rendered visuals relative to the real world, and hence see spurious object-relative motion. This work focuses upon non-see-through HMDs, and thus I explore only subject-relative cues (See Section 4.1.1).

2.2.7.3 Depth Perception Affects Motion Perception

The pivot hypothesis (Gogel, 1990) states that a point stimulus at a distance will appear to move as the head moves if it’s perceived distance differs from its actual distance. A related effect is demonstrated by focusing on a finger held in front of the eyes and noticing that the background further in the distance seems to move with the head. Likewise if one focuses on the background then the finger seems to move against the direction of the head.

Objects in HMDs appear to be closer to users than their intended distance (Loomis and Knapp, 2003). If a user is looking at an object as if it is close, but it moves in the HMD as if it is further away (when turning the head), then, according to the pivot hypothesis, the object will appear to move with the head. In this case, the scene would have to move against the direction of the head turn to appear stable in space.

2.2.7.4 Motion Perception in Peripheral versus Central Vision

The literature conflict as to whether sensitivity to motion increases or decreases with eye eccentricity (the distance from the stimulus on the retina to the center of the fovea). These conflicting claims are likely due to differences of experimental conditions as well as interpretations.

Anstis (1986) states that it is sometimes mistakenly claimed that peripheral vision is more sensitive to motion than central vision. In fact, the ability to detect slow moving

stimuli actually decreases steadily with eye eccentricity. However, since sensitivity to static detail decreases even faster, peripheral vision is relatively better at detecting motion than form. A moving object seen in the periphery is perceived as something moving, but it is more difficult to see what that something is.

Coren et al. (1999) states that the detection of movement depends both on the speed of the moving stimulus and eye eccentricity. A person’s ability to detect slow-moving stimuli (less than 1.5◦/s) decreases with eye eccentricity, which is consistent with Anstis. For faster-moving stimuli, however, the ability to detect moving stimuli increases with eye eccentricity. These differences are due to the dominance in the periphery of the magnocellular pathway, which consist of transient-response cells. Transient-response cells respond best to fast-changing stimuli. Scene motion in the peripheral visual field is also important in sensing self-motion.

2.2.7.5 Motion Perception During Head Movement

Loose and Probst (2001) found increasing angular velocity of the head significantly suppresses the ability to detect visual motion when the visual motion moves relative to the head. They found no statistically significant effect of head angular acceleration on the ability to detect visual motion. They controlled head velocity to be greater than zero and approximately constant for different amounts of head acceleration, and vice versa. They did not measure the ability to detect visual motion with near-zero head velocity and some head acceleration (i.e., at the start, end, or reversal of head turns). Head acceleration may be an important factor when head velocity is near zero.

Loose and Probst also found subjects could better detect visual motion when the visual motion moved by a lesser amount than head motion compared to when the visual motion moved by a greater amount than head motion.

The moving visual stimuli in their experiment were presented in head-centric coordinates, visual motion was judged object-relative to a head-stabilized target, and the subjects were passively rotated via a rotary chair. These conditions contrast to those conditions in an IVE, where scenes are judged to be moving in world coordinates, judgments are subject-relative, and subjects actively rotate their own heads.

Adelstein et al. (2006) and Li et al. (2006) also showed head motion suppresses perception of visual motion. They used an HMD without head tracking, resulting in the image moving in head-centric coordinates; subjects judged motion relative to the head. In my experiments, I investigate if head motion suppresses perception of motion of images in world-centric coordinates; subjects judge visual motion relative to the

world.

2.2.7.6 Motion Illusions

Motion illusions occur in specific situations of real life and often cause people to mischaracterize the environment. Knowledge of these illusions can help investigators improve understanding of human perception, better design perceptual experiments, interpret experimental results, and better design IVEs.

The Autokinetic Effect

The autokinetic effect is the apparent movement of a single stable point-light in a

homogeneous surround (no object-relative cues are present). This effect occurs even when the head is held still. This is because no efference copy occurs for small and slow eye movements. The brain has no relative cues to judge motion and the movement is ambiguous, i.e., did the movement occur due to the eye moving or due to the point-light source moving? An observer does not know the answer, and the amount and direction of perceived motion varies. The autokinetic effect decreases as the size of the target increases, due to the brain’s assumption that objects taking up a large field of view are stable.

The autokinetic effect suggests that large scenes should be used when measuring the ability to detect subject-relative scene motion and I do so in my experiments. Otherwise, subjects may claim to perceive motion even when no motion occurs.

The Aubert-Fleischl and Filhene Illusions

The Aubert-Fleischl illusion causes a moving object to appear to be slower when one

pursues the object with the eyes than when the eyes and head are stable and the image of the object moves across the retina. This illusion occurs as the object moves in front of a stationary background (i.e., the effect only appears when object-relative cues are present).

The Filhene Illusion causes a stable background to appear to move against the direction

of eye motion when one tracks a moving object with the eyes.

I designed my experiments to include only subject-relative cues by blocking out the stationary real world. Thus, the Aubert-Fleischl and Filhene Illusions should not be a factor in this work.

Motion aftereffects

Motion aftereffects are illusions that occur after one views stimuli that move in the same

direction for at least 30 seconds. After this time, the observer may perceive the motion to slow down or stop completely due to fatigue of motion-detection neurons. When the motion stops or the observer looks away from the moving stimuli to non-moving stimuli, she may perceive motion in the opposite direction as the previously moving stimuli.

thresholds. In order to prevent motion aftereffects, I

• do not present the moving scenes for more than 1.2 seconds,

• randomly move the scenes from left-to-right or right-to-left, and

• present stable reference scenes before and after the moving scenes.

Induced Motion

Induced motion occurs when motion of one object induces the perception of motion in

another object. The moon-cloud illusion is an excellent example of induced motion—one may perceive clouds to be stable and the moon to be moving, when in fact the moon is stable and the clouds are moving. This illusion occurs because object-relative cues tell only how objects move relative to each other, not relative to the world. In such circumstances, the mind assumes smaller objects are more likely to move than larger surround objects.

My experiments include subject-relative cues only, by blocking out the stationary real world. Thus induced motion should not be a factor in this work.

Vection

Vection is an illusion similar to induced motion, but instead causes a perception of self-

motion. If an observer is presented with a steadily moving visual pattern, the observer may feel as if she is moving. If the pattern is moved steadily to the side, she may feel that she is moving or leaning to the opposite side and will compensate by leaning into the direction of the visual pattern. Vection is more likely to occur when a large stimulus is moving and when that stimulus is seen in peripheral vision (rather the stimulus is seen or not in the foveal area of vision). Normally, the observer correctly perceives herself to be stable and the stimulus to be moving before the onset of vection. This delay of vection usually lasts several seconds. However, at stimulus accelerations of less than about 5◦/s2, vection is not preceded by a period of perceived stimulus motion, as it is at higher rates of acceleration (Howard, 1986b). Vection can occur when one is seated in a car and an adjacent stopped car pulls away. One experiences the car one is seated in, which is actually stationary, to move in the opposite direction of the moving car. Vection can occur in virtual environments when the entire scene moves independent of user movement.

Latency in HMDs may also cause vection, due to the visual scene moving in a way that does not correspond to head movements (a cue conflict). The observer may incorrectly attribute the movement of the scene to his own motion.

Vestibular stimulation suppresses vection (Lackner and Teixeira, 1977). Since scene motion due to latency occurs only when one moves the head (stimulating the vestibular system), I suspect vection due to latency is relatively rare.

In dark environments, the eye trades its acuity in space and time for increased light sensitivity. The retina integrates over a longer period of time in the dark, delaying perception of stimuli. The differences in delay between a light and dark environment can be up to 100

ms(Anstis, 1986). This delay produces a lengthening of reaction-time for automobile drivers in dim light (Gregory, 1973). Given that visual delay varies for different amounts of dark adaptation or stimulus intensity, then why do people not perceive the world to be unstable or experience motion sickness when they experience greater delays in the dark or with sunglasses as they do in delayed HMDs? I can think of two possible answers to this question:

• The brain might recognize stimuli to be darker and calibrate for the delay appropriately. If this is true, this suggests users can adapt to latency in HMDs. Furthermore, once the brain understands the relationship between wearing an HMD and delay, position constancy could exist for both the delayed HMD and the real world; the brain would know to expect more delay when an HMD is being worn. However, the brain has had a lifetime of correlating dark adaptation and/or stimulus intensity to delay. The brain may take years to correlate the wearing of an HMD to delay.

• The delay inherent in dark adaptation and/or low intensities is not a single precise single delay but the average delay of the integrated signals from a stimulus. This imprecise delay results in motion smear (Coren et al., 1999) and makes precise localization of moving objects more difficult. Perhaps observers are biased to perceive objects to be more stable in such dark situations, but not for the case of brighter HMDs. If this is true, perhaps darkening the scene or adding motion blur to the scene would cause it to appear to be more stable and reduce simulator sickness (although computing motion blur typically adds latency unless prediction is used).